Apparatus for recording and reproducing information on and from an optical disk

ABSTRACT

An apparatus for recording and reproducing information on and from an optical disk is disclosed in which at least one optical beam and a reference clock signal are used. The optical disk comprises recording tracks which are divided into a plurality of blocks and concentrically arranged along the radial direction. The apparatus comprises: a block judging unit for judging one of the blocks that the one block is currently impinged by the optical beam; a clock signal generator for generating a plurality of clock signals which are different in frequency from each other; and a clock signal selecting unit for selecting one of the clock signals as the reference clock signal, on the basis of the judgement of the block judging unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

This invention relates to an apparatus for recording and reproducinginformation on and from an optical disk using an optical beam, and anoptical disk for storing information useful in such an apparatus. In thespecification and accompanying claims, the term "an optical disk"includes an optical disk in which information cannot be rewritten (e.g.,a so-called compact disk) and also that in which information can berewritten (e.g., a magnetooptical disk), and the term "an apparatus forrecording and reproducing information" means an apparatus which canrecord and/or reproducing information on and from an optical disk.

2. Description of the Related Art:

In an apparatus for recording and reproducing information on and from anoptical disk using an optical beam, tracking servo control is performedso that an optical beam such as a laser beam accurately traces recordingtracks preformed in the optical disk. In order to perform the trackingservo control, the continuous tracking servo method is widely employedwhich uses servo control signals obtained from guiding grooves or datapits preformatted in an optical disk for guiding a laser beam along therecording tracks.

An optical disk used in the continuous servo method has preformedcontinuous grooves or data pit rows so that the servo control signal canbe easily obtained from the grooves or pit rows independently of thedata recording density or recording method. Therefore, this method canbe applied to optical disks with various formats.

When using an optical disk on which data is recorded based on variationsin the reflectivity of the recording medium or the presence and absenceof pits, however, the continuous servo method does suffer fromdeterioration of the servo control signal obtained from the grooves orpit rows near the recording areas, due to the effect of the recordedsignal. Furthermore, the quality of the servo control signal tends to beaffected by slight dimensional inaccuracies such as shape errors in theguide grooves. Therefore, it is difficult to obtain compatibilitybetween various disk types.

As another method for performing the tracking servo control, the sampleservo method is employed which uses a tracking servo control signalobtained from servo bytes preformatted in sampling areas which areformed locally on the optical disk. In the sample servo method, a phaselock loop (PLL) circuit generates a reference clock signal insynchronization with the timing with which an optical beam passes theservo bytes. Based on this reference clock signal, a tracking servocontrol signal is obtained from the servo bytes, and the timing withwhich the data signal is written and/or read is controlled.

In the sample servo method, the sampling areas are completely separatedfrom the areas in which data is recorded, and therefore the trackingservo control signal is not affected by the recording signal and atracking servo control signal of high quality can be easily obtained.Furthermore, the formation of sampling areas specialized for thetracking servo control allows the apparatus to use a relatively simpledetection means, resulting in that the sample servo method can offer awider range of disk compatibility than the continuous servo method.

When a focus servo control is performed against an optical disk havingguide grooves in the astigmatic method, the presence of the guidegrooves may cause a shadow to form in a beam spot on an opticaldetector, thereby necessitating the precise positioning of the opticaldetector. Since it is not necessary to form guide grooves in an opticaldisk used in the sample servo method, in contrast, such a shadow doesnot form even when a focus servo control is conducted. This results inthat the servo control can be accurately executed without excessivelyaccurate positioning of an optical detector. Therefore, the sample servomethod offers advantages such as reduced steps in the assembly of thedetector and an improved yield of an apparatus.

Generally, an optical disk is driven under either the constant angularvelocity (CAV) control in which the rotational velocity of the disk iscontrolled so that its angular velocity is kept constant, or theconstant linear velocity (CLV) control in which the rotational velocityof the disk is controlled so that the speed of the optical beam spotwith respect to the optical disk (i.e., the linear velocity) ismaintained constant.

While the CAV control is simple since the rotational velocity is keptconstant irrespective of the irradiation position of the optical beam onthe optical disk, the CAV control involves a disadvantage in that therecording density at the portion nearer the outer circumference of thedisk becomes smaller, thus reducing the storage capacity of the disk asa whole.

In the CLV control, on the other hand, the storage capacity can beeasily increased, but the rotational velocity of the optical disk mustbe varied in accordance with the change of the radial position of theoptical beam spot on the optical disk. Moreover the extra time requiredfor stabilizing the rotational velocity makes the access time longer.

To tackle these problems, the modulate-constant angular velocity (M-CAV)control has been proposed in which the rotational velocity of theoptical disk is kept constant, the recording area of the disk is dividedinto a plurality of blocks comprising multiple tracks, and informationis written and/or read using a clock signal having a higher frequency inthe more outer block. According to this M-CAV control, it is possible toavoid the increase in the access time caused by the change of therotational velocity, and in addition, the linear recording density nearthe outer circumference of the optical disk is not reduced, so thestorage capacity can be easily increased.

However, in the M-CAV control, it is difficult to employ the sampleservo method which has various advantages as described above, because ofthe reasons mentioned below.

That is, in the M-CAV control, the period with which the optical beampasses a servo byte changes with each block. Therefore, when a sampleservo method is used, each time the optical beam moves in the radialdirection of the optical disk and enters into another block, thetracking control cannot be correctly performed until the PLL circuit ispulled again into synchronism.

In the sample servo method, comparison pulses input to the PLL circuitper unit time is small in number so that, if the period with which theoptical beam passes the servo byte changes greatly, the response time isprolonged until the PLL circuit is pulled again into synchronization togenerate a stable reference clock signal. Consequently, even though theM-CAV control method does not require the change of the rotationalvelocity of the optical disk, it does result in longer access times.

When performing the M-CAV control in the sample servo method, therefore,it is difficult to shorten the overall access time while increasing thestorage capacity of an optical disk.

SUMMARY OF THE INVENTION

According to the invention, an apparatus for recording and reproducinginformation on and from an optical disk, using at least one optical beamand a reference clock signal, said optical disk comprising recordingtracks which are divided into a plurality of blocks, said blocks beingconcentrically arranged along the radial direction is provided, whichovercomes the above-discussed and numerous other disadvantages anddeficiencies of the prior art, the apparatus comprising: a block judgingmeans for judging one of said blocks that said one block is currentlyimpinged by said optical beam; a clock signal generating means forgenerating a plurality of clock signals which are different in frequencyfrom each other; and a clock signal selecting means for selecting one ofsaid clock signals as said reference clock signal, on the basis of thejudgment of said block judging means.

In a preferred embodiment, the apparatus further comprises a motherclock signal generating means for generating a mother clock signal, saidmother clock signal being supplied to said clock signal generatingmeans.

In a preferred embodiment, the mother clock signal generating means is aphase lock loop circuit.

In a preferred embodiment, the clock signal generating means comprises aplurality of frequency dividers for dividing said mother clock signal,the outputs of said frequency dividers being supplied to t said clocksignal selecting means as said clock signals.

In a preferred embodiment, the apparatus further comprises a reset meansfor resetting said frequency dividers.

In a preferred embodiment, the apparatus further comprises a drivingmeans for rotating said optical disk at a predetermined angularvelocity.

According to the invention, an optical disk for storing information isprovided, comprising a plurality of blocks which are separatedconcentrically in the radial direction, each of said blocks including atleast one recording tracks elongating tangentially, each of saidrecording tracks comprising a plurality of segments, each of saidsegments including a sampling area and a data region for storinginformation, said sampling area containing preformatted servo bytes,each of segments of tracks in the same block having the same centralangle, the number of segments included in one track belonging to anouter block being greater than that of segments included in one trackbelonging to an inner block.

In a preferred embodiment, the mean length of segments belonging to oneof said blocks is substantially the same as the mean lengths of segmentsbelonging to other blocks.

In a preferred embodiment, one of said sampling areas in each saidtracks is positioned on the same radius of said optical disk.

In a preferred embodiment, two or more of said sampling areas in eachsaid tracks are positioned respectively on two or more radii of saidoptical disk.

Thus, the invention described herein makes possible the objectives of:

(1) providing an apparatus for recording and reproducing information onand from an optical disk which can increase the storage capacity of theoptical disk and prevent the increase of the overall access time evenwhen the M-CAV control is performed in the sample servo method;

(2) providing an apparatus for recording and reproducing information onand from an optical disk in which the reference clock signal can bequickly stabilized after the optical beam jumps from one block of theoptical disk to another block;

(3) providing an apparatus for recording and reproducing information onand from an optical disk in which the servo control and the informationreproduction and/or record can be quickly resumed after the optical beamjumps from one block of the optical disk to another block; and

(4) providing an optical disk which can be used under the M-CAV controlin the sample servo method, with increasing the storage capacity of theoptical disk and preventing the increase of the overall access time.

By means of the above configuration, after the optical beam moves orjumps from a block to another block in the radial direction of theoptical disk, the new block currently being irradiated by the opticalbeam is rapidly discriminated by the block judging means. Hereinafter,such a block currently irradiated by the optical beam is referred to as"a current block". Then, the clock signal selecting means selects theclock signal with the frequency corresponding to the current block amongthe clock signals generated by the clock signal generating means, basedon the signal from the block judging means.

That is, even when the optical beam moves to another block, a stabilizedreference clock signal can be obtained in a short time, so that theservo control and the writing and/or reading of information can beresumed quickly. Therefore, even if the linear recording density nearthe outer edge of the disk is increased by performing the M-CAV controlwhile using the sample servo method, the overall access time is notincreased.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention may be better understood and its numerous objects andadvantages will become apparent to those skilled in the art by referenceto the accompanying drawings as follows:

FIG. 1 is a block diagram showing an apparatus according to theinvention.

FIG. 2 illustrates an optical disk according to the invention.

FIG. 3 is a diagram showing diagrammatically the structure of tracksformed in the optical disk of FIG. 2.

FIG. 4 is a timing chart of the synchronizing pulse signal produced inthe apparatus of FIG. 1.

FIG. 5 is a block diagram showing a block judging unit used in theapparatus of FIG. 1.

FIG. 6 is a timing chart illustrating the operation of the judging unitof FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows an optical disk according to the invention. In the opticaldisk 11 of FIG. 2, the recording area is divided into three blocks 14,15 and 16 each having a plurality of concentric tracks 12. Namely,tracks 12 formed between radii R₁ -R₂ constitute the first block 14.Likewise, tracks 12 formed between radii R₂ -R₃ and between R₃ -R₄constitute the second and third blocks 15 and 16, respectively (R₄ >R₃>R₂ >R₁).

Each track 12 is divided into segments 13 each comprising a seriesarrangement of a sampling area 13a for generating the servo signal and adata area 13b for recording data. As shown in FIG. 3, the sampling areas13a have a preformatted servo byte consisting of wobbled pits 21 forgenerating the tracking signal, and a synchronizing pit 22 forgenerating the synchronizing pulse. The tracking signal is used as theservo control signal. In the data areas 13b, data pits 23 for recordinginformation are formed by a suitable means such as engraved pits,magnetically inverted patterns, etc. The wobbled pits 21 andsynchronizing pits 22 are positioned according to the internationalstandard ISO-DP9171-2.

In one block, the numbers of the segments 13 included in each of thetracks 12 are identical with each other. That is, in the same block, thesegments 13 have the same central angle, so that the period with whichthe sampling areas 13a are irradiated by the optical beam is the samefor any track in the same block when the optical disk 11 is rotated at aconstant angular velocity.

In contrast, the number of the segments 13 belonging to one of theblocks differs from that of the segments 13 belonging to another of theblocks. More specifically, the numbers of the segments 13 are set sothat tracks 12 in the outer blocks have more segments (i.e., the numberof the segments in the block 16 is greater than that in the block 14).In other words, the mean length of the segments 13 belonging to one ofthe blocks 14-16 is substantially the same as that of the segmentsbelonging to other one of the blocks 14-16, so that the linear recordingdensity is approximately constant among the blocks 14-16.

In the embodiment, the positions of the segments 13 are set in such amanner that the sampling areas 13a of one of the segments 13 in each ofthe tracks 12 is positioned on the same or common radius A, therebyenabling the synchronization between the blocks 14-16 to be easilyconducted. According to this arrangement, moreover, the synchronizingpits 22 can be readily detected even when the optical beam is traversingthe tracks 12.

FIG. 1 shows an apparatus according to the invention which can recordand reproduce information on and from the above-described optical disk11. The apparatus of FIG. 1 comprises a block judging unit 31, a PLLcircuit 32, a clock signal generator 44, a reference clock signalselection unit 45, and a reset signal generator 46. The PLL circuit 32includes a phase comparator 33, a low-pass filter 34, a voltagecontrolled oscillator (VCO) 35, three PLL clock signal frequencydividers 36, 37 and 38, and a PLL clock signal selector 39. The clocksignal generator 44 has three reference clock signal frequency dividers41, 42 and 43.

The optical disk 11 is mounted on the apparatus, and scanned by anoptical beam emitted from a semiconductor laser device (not shown) whilebeing rotated at a predetermined constant angular velocity by a suitabledriving device (not shown). The optical beam is reflected from thewobbled pits 21, synchronizing pits 22 and data pits 23 of the opticaldisk 11. The reflected beams are detected to obtain a reproduced signalS from the optical disk. The means for performing the CAV control andthe manner of obtaining the reproduced signal is well known in the art,and therefore their detailed description is omitted.

The reproduced signal S is supplied to the block judging unit 31. Theblock judging unit 31 also receives clock signals CK(A), CK(B) and CK(C)from the reference clock signal frequency dividers 41-43. The blockjudging unit 31 produces a synchronizing pulse signal SP and a blockjudging signal BJ. The synchronizing pulse signal SP is input to thephase comparator 33 of the PLL circuit 32. The phase comparator 33compares the phase of the synchronizing pulse signal SP with that of aPLL clock signal input from the PLL clock signal selector 39, andoutputs a phase error signal PE. The phase error signal PE is input tothe low-pass filter 34 which in turn outputs a control voltage having alevel corresponding to the phase difference between the synchronizingpulse signal SP and the input PLL clock signal.

The control voltage is input to the VCO 35 which generates a motherclock signal CK(M) with a frequency corresponding to the level of theinput control voltage and synchronized with the synchronizing pulsesignal SP. The mother clock signal CK(M) is supplied to the PLL clocksignal frequency dividers 36-38 and also to the reference clock signalfrequency dividers 41-43 of the clock signal generator 44.

The PLL clock signal frequency dividers 36-38 divide the mother clocksignal CK(M) to respectively generate clock signals CL(A), CL(B) andCL(C). Each of the clock signals CL(A)-CL(C) has a frequencyapproximately equal to that of the synchronizing pulse signal SP whichis produced from the block judging unit 31 when the corresponding one ofthe blocks 14-16 of the optical disk 11 is irradiated by the opticalbeam. The clock signals CL(A)-CL(C) are input to the PLL clock signalselector 39. The PLL clock signal selector 39 selects one of the clocksignals CL(A)-CL(C) on the basis of the block judging signal BJ outputfrom the block judging unit 31, and then sends out the selected clocksignal to the phase comparator 33 as the PLL clock signal.

As mentioned above, the mother clock signal CK(M) is also supplied tothe reference clock signal frequency dividers 41-43 of the clock signalgenerator 44. The frequency dividers 41-43 divide the mother clocksignal CK(M) to respectively generate clock signals CK(A), CK(B) andCK(C). Each of the clock signals CK(A)-CK(C) has a frequencycorresponding to the period with which the sampling areas 13a in each ofthe blocks 14-16 are irradiated by the optical beam. The clock signalsCK(A)-CK(C) are supplied to the block judging unit 31 as mentionedabove, and also to the reference clock signal selection unit 45. Amongthese clock signals CK(A)-CK(C), the one having the frequencycorresponding to the current block is selected by the reference clocksignal selection unit 45. The selected clock signal is output as areference clock signal CK which is employed for performing the servocontrol, and the record and reproduction of information.

The output of the reset signal generator 46 is connected to the PLLclock signal frequency dividers 36-38 and to the reference clock signalfrequency dividers 41-43. When the optical beam irradiates one of thesegments 13 which is positioned on the radius A, a control unit (notshown) produces in a conventional manner a reset control signal, andsupplies it to the reset signal generator 46. Upon receiving the resetcontrol signal, the reset signal generator 46 generates a reset signalso that the counters of the frequency dividers 36-38 and 41-43 can bereset and the outputs of the frequency dividers can be synchronized.

The manner of generating the synchronizing pulse signal SP and the blockjudging signal BJ in the block judging unit 31 will be described withreference to FIG. 5. The block judging unit 31 comprises: asynchronizing pit detector 51 to which the reproduced signal S is input;window generators 52A-52C which respectively receive the clock signalsCK(A)-CK(C); AND gates 53A-53C, an OR gate 54; and a judging circuit 55.

As shown in FIG. 6, the window generators 52A-52C, which may becounters, generate respectively synchronizing pit extracting windowsignals WA, WB and WC for T sec. when the periods of time T_(A), T_(B)and T_(C) have elapsed after the reset (T_(A) <T<T_(C), and T_(B)-T_(A) >T, T_(C) -T_(B) >T). In the embodiment, the synchronizing pulsesignal SP is supplied also to the window generators 52A-52C, to functionas a reset signal. The window signals WA-WC are input respectively tothe AND gates 53A-53C.

The synchronizing pit detector 51 produces from the reproduced signal Sa synchronizing pit detection signal DP which indicates that thesynchronizing pit 22 in the current block is irradiated by the opticalbeam. The synchronizing pit detection signal DP is input to the ANDgates 53A-53C. The outputs DA-DC of the AND gates 53A-53C are input tothe OR gate 54. When the optical beam irradiates the block 14 (i.e., theblock 14 is the current block), the synchronizing pit detection signalDP appears at the timing corresponding to the window signal WA,resulting in that the output DA of the AND gate 53A becomes HIGH whilethe outputs DB and DC of the AND gates 53B and 53C remain LOW. When theoptical beam irradiates the block 15 or 16 (i.e., the block 15 or 16 isthe current block), in contrast, the synchronizing pit detection signalDP appears at the timing corresponding to the window signal WB or WC, asshown by the broken line in FIG. 6, and the output DB or DC becomesHIGH. The output DA is output from the OR gate 54 as the synchronizingpulse signal SP.

In this way, the synchronizing pulse signal SP is obtained in which thetiming of HIGH-period depends on the current block, as shown in FIG. 4.That is, the linear recording density of each track 12 is approximatelythe same in the blocks 14-16, and the optical disk 11 is rotated underthe CAV control so that the period of the synchronizing pulse signal SPobtained from an outer block is shorter than that obtained from an innerblock. Since one sampling area 13a of one of the segments 13 of eachtrack 12 is on the same radius A, moreover, the synchronizing pulsesignal SP is obtained with the same timing once each revolution of theoptical disk 11, irrespective of the position of the optical beam (i.e.,whichever block is the current block). FIG. 4 shows the synchronizingpulse signals SP obtained from the blocks 14-16.

The outputs DA-DC of the AND gates 53A-53C are also input to the judgingcircuit 55. The judging circuit 55 judges that, when the output DA isHIGH, the block 14 is the current block, and similarly that, when theoutput DB or DC is HIGH, the block 15 or 16 is the current block.According to these judgments, the judging circuit 55 produces the blockjudging signal BJ indicating the identification of the current block.

The operation of the apparatus of FIG. 1 will be described in moredetail. In the PLL circuit 32 which receives the synchronizing pulsesignal SP, the VCO 35 generates the mother clock signal CK(M) having afrequency which is an integral multiple of that of the synchronizingpulse signal SP and synchronized with the signal SP. Three clock signalsCL(A)-CL(C) are produced from the mother clock signal CK(M) by the PLLclock signal frequency dividers 36-38. The PLL clock signal selector 39selects in accordance with the input block judging signal BJ one of theclock signals CL(A)-CL(C) which has the same frequency as thesynchronizing pulse signal SP. The selected PLL clock signal is input tothe phase comparator 33. The phase comparator 33 generates the phaseerror signal PE corresponding to the phase difference between the inputPLL clock signal and the synchronizing pulse signal SP. In the VCO 35 towhich the control voltage is supplied from the low-pass filter 34, thefrequency of the mother clock signal CK(M) is controlled according tothe level of the input control voltage, and then the oscillationfrequency of the VCO 35 is controlled by the control voltage so that theabove-mentioned phase difference becomes null.

The mother clock signal CK(M) is also supplied to the clock signalgenerator 44 from which the clock signals CK(A)-CK(C) are generated. Thefrequencies of the clock signals CK(A)-CK(C) respectively correspond tothe periods with which the sampling areas 13a in each of the blocks14-16 are irradiated by the optical beam. More specifically, thefrequencies of the clock signals CK(A)-CK(C) are respectively dividedinto the sampling frequencies of the wobbled pits 21, synchronizing pits22 and data pits 23 in the blocks 14-16 (i.e., to the frequencies whichare integral multiples of the synchronizing pulse signal SP).

In accordance with the block judging signal BJ from the block judgingunit 31, the reference clock signal selection unit 45 selects one of theclock signals CK(A)-CK(C) which has a frequency corresponding to thecurrent block. The selected clock signal is output as the referenceclock signal CK, and is used to perform the servo control and theinformation record and/or reproduction operation against the currentblock.

According to the invention, no matter which block is the current block,the clock signals CK(A)-CK(C) having a frequency correspondingrespectively to the blocks 14-16 are always generated from the clocksignal generator 44. Based on the block judging signal BJ, the referenceclock signal selection unit 45 selects the clock signal with a frequencycorresponding to the current block, from the clock signals CK(A)-CK(C),and outputs it as the reference clock signal CK. When the optical beammoves radially on the optical disk 11 from one block to another, theblock judging means 31 immediately know the new current block, i.e., thecontents of the block judging signal BJ output from the block judgingmeans 31 are rapidly changed to indicate the new current block, with theresult that the stabilized clock signal CK having a frequencycorresponding to the current block can be quickly obtained. Therefore,the servo control and the information record and/or reproductionoperation can be performed rapidly. Moreover, the transient response ofthe PLL circuit can be suppressed by the switching of the PLL clocksignal selector 39, thereby further shortening the time required formoving the optical beam

According to the invention, therefore, the storage capacity of anoptical disk can be increased using the sample servo method andperforming the M-CAV control without increasing the overall access time.

In another embodiment, the PLL clock signal frequency dividers 36-38 andthe PLL clock signal selector 39 are not provided, and a clock signalwhich is obtained by dividing the reference clock signal CK is suppliedto the phase comparator 33 of the PLL circuit 32. The manner of dividingtracks of an optical disk into blocks and the number of clock signalfrequency dividers are not restricted to the above, and may beadequately selected to comply with the object of the system. In anotheroptical disk according to the invention, two or more common radii A maybe provided so that the sampling areas 13a of two or more segments 13 ineach track 12 are respectively positioned on the radii A.

It is understood that various other modifications will be apparent toand can be readily made by those skilled in the art without departingfrom the scope and spirit of this invention. Accordingly, it is notintended that the scope of the claims appended hereto be limited to thedescription as set forth herein, but rather that the claims be construedas encompassing all the features of patentable novelty that reside inthe present invention, including all features that would be treated asequivalents thereof by those skilled in the art to which this inventionpertains.

What is claimed is:
 1. A servo control apparatus for a device whichrecords information on or reads information from an optical disk havinga plurality of concentric blocks including plural sample areas and dataregions, comprising:controlling means for controllably directing anoptical beam to impinge upon a location on said optical disk in responseto a reference clock signal; block judging means for judging which ofsaid plurality of concentric blocks said optical beam is impinging upon;clock signal generating means for generating a plurality of clocksignals simultaneously, each of said plurality of clock signals having adifferent frequency corresponding to the period that different samplingareas of each of the different blocks are impinged upon by said opticalbeam; clock signal selecting means for selecting said reference clocksignal from said plurality of clock signals generated by said clocksignal generating means on the basis of which block said block judgingmeans has judged said optical beam to be impinging upon; and means forproviding said reference clock signal to said controlling means.
 2. Anapparatus according to claim 1, wherein said apparatus further comprisesa mother clock signal generating means for generating a mother clocksignal, said mother clock signal being supplied to said clock signalgenerating means.
 3. An apparatus according to claim 2, wherein saidmother clock signal generating means is a phase lock loop circuit.
 4. Anapparatus according to claim 2, wherein said clock signal generatingmeans comprises a plurality of frequency dividers for dividing saidmother clock signal, the outputs of said frequency dividers beingsupplied to said clock signal selecting means as said clock signals. 5.An apparatus according to claim 4, wherein said apparatus furthercomprises a reset means for resetting said frequency dividers.
 6. Anapparatus according to claim 1, wherein said apparatus further comprisesa driving means for rotating said optical disk at a predeterminedangular velocity.
 7. A servo control apparatus for control of recordingand reproducing information transferred onto and from an optical diskvia an optical signal, the optical disk having plural concentric blockareas, each block having at least one concentric track and each trackbeing divided into plural segments with each segment having a samplingarea and a data area, comprising:means for generating simultaneouslyplural reference clock signals having different frequencies associatedwith a block's radial position on the optical disk and to periods whenthe optical signal impinges on sampling areas in a particular block;means for receiving the plural reference clock signals and aninformation signal corresponding to the optical signal; means fordetermining a current block upon which the optical signal currentlyimpinges and generating a current block signal; means for selecting oneof the reference clock signals based on the current block signal; andmeans for providing the selected clock signal for control of the opticalsignal.
 8. The apparatus according to claim 7, wherein the means fordetermining also generates a synchronization signal and the apparatusfurther comprises:a phase-locked loop (PLL) circuit including: avoltage-controlled oscillator for generating a base clock signal; aplurality of PLL clock dividers for generating different PLL clocksignals from the base clock signal; a PLL clock selector for selectingone of the PLL clock signals based on the current block signal; and aphase comparator for comparing the selected PLL clock signal with thesynchronization signal and generating an error signal for controllingthe voltage-controlled oscillator.
 9. An apparatus according to claim 8,wherein each block requires different frequency synchronization signalsbased on the number of segments in each block, all of thesynchronization signals aligned with and referenced from a common radiuson the optical disk.
 10. The apparatus according to claim 7, wherein thedetermining means includes:a detector for detecting a blocksynchronization signal in the information signal, and plural logicgates, each logic gate receiving the detected synchronization signal anda window pulse signal corresponding to one of the reference clocksignals and selecting its corresponding reference clock signal when thedetected synchronization signal is received during the correspondingwindow pulse signal.
 11. The apparatus according to claim 7, wherein thenumber of segments included in each track of an outer block is greaterthan the number of segments included in each track of an inner block,the inner block having a smaller radial position on the optical diskrelative to the outer block.
 12. The apparatus according to claim 7,wherein a mean length of each segment in each block is substantiallyequal.
 13. The apparatus according to claim 7, wherein one of thesampling areas in each track of all blocks is positioned on the sameradius of the optical disk.